First of all i would like to point you to
This Section of the Forums for any future Questions you may have.
Now as for this Question, When you logout your car is de-spawned, when you log in next time the car re-spawns where ever you did "/car park" If you didn't do this command it will be either at Mullholland Intersection, Or wt the car dealership you brought it from.
If you wish to locate the car you may do the following things to find it:
1. /myvehicles - This will bring up a list of your cars find the car you are looking for and look for it's ID
For Example My car ID is 1519
When you get the ID Type
/car find [Car ID]
This will prompt you and give you a red beacon to drive/run to to find your car.
When you find your car go to where you're renting park the car and /car park If you don't rent i suggest finding a nice place near Union Station to park your car.